This invention relates to the field of drafting and more particularly to drafting devices for use in making perspective drawings.
Architects, engineers and the like, who often need to make perspective drawings, generally need to utilize vanishing points that are located considerable distances outside the edges of the drawing board, often as much as 8 to 10 feet or more outside the drawing board edge, to provide the general size and realistic detail of the drawing being made.
Although a number of devices have been developed to assist drafters to make perspective drawings, none of these have become generally available to architects and others who make perspective drawings, due to limitations in providing suitable vanishing points at various distances outside the drawing board, complicated methods of utilization, and high acquisition cost due to complicated mechanisms.
Some of such previous art includes U.S. Pat. Nos. 3,492,727 by J. Klok, 3,464,117 by J. Johnson, 4,137,635 by D. Harvey and 3,389,470 by J. Johnson.
Others have utilized a combination of mechanical and electronic devices making the cost of these prohibitive for general use. Some have required permanent fastening to a drawing board, limiting portability and rapid changing of orientation on the drafting surface.
U.S. Pat. No. 3,672,062 by A. Baker shows a portable drafting device using a magnetized roller that however only will make parallel lines.
U.S. Pat. No. 4,586,265 by W. Hopkins shows a perspective drawing apparatus utilizing two wheels of different diameter on a common axle to provide some vanishing points in or outside the board by varying the distance between the wheels. This device will not provide a vanishing point that is infinitely far away to make parallel lines and requires that the drafter applies constant pressure to the device on the drawing board to prevent loss of orientation to a selected vanishing point. Vanishing point can not be switched from left to right and only one vanishing point can be set at a time.
Generally, whenever possible, vanishing points are chosen to lie within the boundaries of the drafting board, where they may be easily marked, for instance by a pin pressed into the board, against which a ruler is located and pivoted around to provide means for drawing perspective lines converging to such a vanishing point. Often much greater distance is required to the vanishing point, causing it to lie outside the edge of the drafting board, sometimes many feet away.